Selectivated zeolite catalyst treated with a dealuminizing agent

ABSTRACT

There is provided a zeolite catalyst, which is first selectivated with a siliceous material and then treated with an aqueous solution comprising a dealuminizing agent such as an acid or a chelating agent.

BACKGROUND

There is provided a zeolite catalyst, which is first selectivated with asiliceous material and then treated with an aqueous solution comprisingwith a dealuminizing agent such as an acid or a chelating agent.

Shape-selective catalysis is described, e.g., by N. Y. Chen, W. E.Garwood, and F. G. Dwyer, Shape Selective Catalysis in IndustrialApplications, 36, Marcel Dekker, Inc. (1989). Within a zeolite pore,hydrocarbon conversion reactions such as isomerization,disproportionation, alkylation, and transalkylation of aromatics aregoverned by constraints imposed by the channel size. Reactantselectivity may occur when a fraction of the feedstock is too large toenter the zeolite pores to react, while product selectivity may occurwhen some of the products cannot leave the zeolite channels. Productdistributions can also be altered by transition state selectivity inwhich certain reactions cannot occur because the reaction transitionstate is too large to form within the zeolite pores or cages. Anothertype of selectivity results from configurational constraints ondiffusion where the dimensions of the molecule approach that of thezeolite pore system. A small change in the dimensions of the molecule orthe zeolite pore can result in large diffusion changes leading todifferent product distributions. This type of shape-selective catalysisis demonstrated, for example, in selective alkyl-substituted benzenedisproportionation to para-dialkyl-substituted benzene.

A representative para-dialkyl-substituted benzene is para-xylene. Theproduction of para-xylene may be performed by methylation of toluene orby toluene disproportionation over a catalyst under conversionconditions. Examples include the reaction of toluene with methanol, asdescribed by Chen et al., J. Amer. Chem. Soc., 101, 6783 (1979), andtoluene disproportionation, as described by Pines in The Chemistry ofCatalytic Hydrocarbon Conversions, Academic Press, 72 (1981). Suchmethods may result in the production of a mixture of the three xyleneisomers, i.e., para-xylene, ortho-xylene, and meta-xylene. Dependingupon the degree of selectivity of the catalyst for para-xylene(para-selectivity) and the reaction conditions, different percentages ofpara-xylene are obtained. The yield, i.e., the amount of xylene producedas a proportion of the feedstock, is also affected by the catalyst andthe reaction conditions.

Various methods are known in the art for increasing the para-selectivityof zeolite catalysts. One such method is to modify the catalyst bytreatment with a "selectivating agent." For example, U.S. Pat. Nos.5,173,461; 4,950,835; 4,927,979; 4,465,886; 4,477,583; 4,379,761;4,145,315; 4,127,616; 4,100,215; 4,090,981; 4,060,568; and 3,698,157disclose specific methods for contacting a catalyst with a selectivatingagent containing silicon ("silicon compound"),

U.S. Pat. No. 4,548,914 describes another modification method involvingimpregnating catalysts with oxides that are difficult to reduce, such asthose of magnesium, calcium, and/or phosphorus, followed by treatmentwith water vapor to improve para-selectivity.

European Patent No. 296,582 describes the modification ofaluminosilicate catalysts by impregnating such catalysts withphosphorus-containing compounds and further modifying these catalysts byincorporating metals such as manganese, cobalt, silicon and Group IIAelements. The patent also describes the modification of zeolites withsilicon compounds.

U.S. Pat. No. 4,283,306 to Herkes discloses the promotion of crystallinesilica catalyst by application of an amorphous silica such asethylorthosilicate (i.e., tetraethylorthosilicate). The Herkes patentcontrasts the performance of catalyst treated once with anethylorthosilicate solution followed by calcination against theperformance of catalyst treated twice with ethylorthosilicate andcalcined after each treatment. The Herkes disclosure shows that thetwice-treated catalyst is less active and less selective than theonce-treated catalyst as measured by methylation of toluene by methanol,indicating that the multiple ex situ selectivation confers no benefitand in fact reduces a catalyst's efficacy in shape-selective reactions.

Steaming has also been used in the preparation of zeolite catalysts tomodify the alpha or improve stability. For example, U.S. Pat. No.4,559,314 describes steaming a zeolite/binder composite at 200°-500° C.for at least an hour to enhance activity by raising the alpha. U.S. Pat.No. 4,522,929 describes pre-steaming a fresh zeolite catalyst so thatthe alpha activity first rises then falls to the level of the freshunsteamed catalyst, producing a stable catalyst which may be used inxylene isomerization. U.S. Pat. No. 4,443,554 describes steaminginactive zeolites (Na ZSM-5) to increase alpha activity. U.S. Pat. No.4,487,843 describes contacting a zeolite with steam prior to loadingwith a Group IIIB metal.

Various organic compounds have been employed as carriers for siliconcompounds in the silicon impregnation methods applied to zeolitecatalysts. For example, U.S. Pat. Nos. 4,145,315; 4,127,616; 4,090,981;and 4,060,568 describe the use of inter alia C₅₋₇ alkanes as solventsfor silicon impregnation.

In accordance with U.S. Pat. No. 4,503,023, aluminum from AlO₄-tetrahedra of zeolites is extracted and substituted with silicon toform zeolite compositions having higher SiO₂ /Al₂ O₃ molar ratios. Theobject of the aluminum extraction process of U.S. Pat. No. 4,503,023 isto increase the silica to alumina ratio of a large pore zeolite catalysthaving a relatively fragile crystal framework, notably zeolite Y,without damaging or destroying the crystal. The preparative procedureinvolves contact of a starting zeolite having a SiO₂ /Al₂ O₃ molar ratioof about 3 or greater with an aqueous solution of a fluorosilicate saltusing controlled proportions and temperature and pH conditions which areintended to avoid aluminum extraction without silicon substitution. Thefluorosilicate salt serves as the aluminum extractant and as the sourceof extraneous silicon which is inserted into the zeolite structure inplace of the extracted aluminum.

U.S. Pat. No. 4,427,790 describes a process for improving the activityof crystalline zeolites in which the zeolite in the "as synthesized"form or following ion-exchange is reacted with a compound having acomplex fluoranion.

The use of chelating agents to remove framework and non-frameworkaluminum from faujasite material is shown by G. T. Kerr, "Chemistry ofCrystalline Aluminosilicates. v. Preparation of Aluminum DeficientFaujasites", J. Phys. Chem. (1968) 72 (7) 2594; T. Gross et al.,"Surface Composition of Dealuminized Y Zeolites Studied by X-RayPhotoelectron Spectroscopy", Zeolites (1984) 4, 25; and J. Dwyer et al.,"The Surface Composition of Dealuminized Zeolites", J. Chem. Soc., Chem.Comm. (1981) 42.

Other references teaching removal of aluminum from zeolites include U.S.Pat. No. 3,442,795 and U.K. Patent No.. 1,058,188 (hydrolysis andremoval of aluminum by chelation); U.K. Patent No. 1,061,847 (acidextraction of aluminum); U.S. Pat. No. 3,493,519 (aluminum removal bysteaming and chelation); U.S. Pat. No. 4,273,753 (dealuminization bysilicon halides and oxyhalides); U.S. Pat. No. 3,691,099 (aluminumextraction with acid); U.S. Pat. No. 4,093,560 (dealuminization bytreatment with salts); U.S. Pat. No. 3,937,791 (aluminum removal withCr(III) solutions); U.S. Pat. No. 3,506,400 (steaming followed bychelation); U.S. Pat. No. 3,640,681 (extraction of aluminum withacetylacetonate followed by dehydroxylation); U.S. Pat. No. 3,836,561(removal of aluminum with acid); German Patent No. 2,510,740 (treatmentof zeolite with chlorine of chlorine-containing gases at hightemperatures); Netherlands Patent No. 7,604,264 (acid extraction), JapanPatent No. 53,101,003 (treatment with EDTA or other materials to removealuminum) and J. Catalysis, 54, 295 (1978) (hydrothermal treatmentfollowed by acid extraction).

SUMMARY

There is provided a method for preparing a selectivated catalystcomposition, said method comprising the steps of:

(a) contacting a catalyst comprising a zeolite with an organosiliconcompound under conditions sufficient to deposit a siliceous material onsaid catalyst;

(b) contacting the catalyst comprising a zeolite and siliceous materialfrom step (a) with an aqueous solution comprising at least onedealuminizing agent;

(c) washing the aqueous solution treated catalyst from step (b) withwater;

(d) drying the washed catalyst from step (c); and

(e) calcining the dried catalyst from step (d) under conditionssufficient to remove any residue of said dealuminizing agent.Optionally, steps (b) and (c) may be repeated at least once beforeconducting step (d).

EMBODIMENTS

In a process for the selective production of para-xylene by thedisproportionation of toluene over a selectivated catalyst, it isdesirable to operate the process in a steady-state fashion, whereinessentially constant levels of toluene conversion and para-selectivityare maintained. In this regard, it will be understood thatpara-selectivity refers to the percentage of para-xylene in the overallmixture of xylene isomers obtained. For example, during such asteady-state phase of operation, the conversion of toluene may vary byonly a small amount, e.g., ± about 2 wt. %, from a target conversionrate, e.g., selected from a particular percentage in the range of fromabout 25% to about 35%, such as about 30%. Similarly, thepara-selectivity may vary by only a small amount, e.g., ± about 2%, froma target para-selectivity, e.g., selected from a particular percentagein the range of from about 85% to about 95%, such as about 91%.

In order to compensate for reduced activity of the catalyst, broughtabout primarily by coking, the conditions of the reaction must beadjusted periodically to maintain the steady-state of operation. Thisadjustment is generally made by incrementally increasing the temperatureof the reaction. For example, the temperature of the reaction may beincreased on a daily basis in an amount sufficient to return thereaction to the target level of conversion. In such a case the averagedaily rate of temperature increase in the reactor provides a measure ofthe aging rate of the catalyst. Depending upon the aging rate of thecatalyst, the steady-state of operation may be maintained for anextended period, e.g., for at least 30 days or even at least 100 days.When the catalyst becomes sufficiently aged such that steady-stateoperation is no longer practical, the reaction may be interrupted andthe catalyst regenerated.

When fresh catalyst is first loaded into the reactor, it may not bepossible or practical to maintain a steady-state of operation during theinitial stages of the reaction. More particularly, it has been observedthat the para-selectivity of toluene disproportionation may increaseduring the initial stages of a reaction, especially when the catalystcomprises ZSM-5 selectivated with a siliceous material. While notwishing to be bound by any theory, it is theorized that a small amountof coke must be deposited on certain catalysts before the targeted levelof para-selectivity is achieved. This initial phase of operation, priorto steady-state operation, is also referred to herein as the adjustmentphase or the line-out period. The para-selectivity of the reaction mayincrease by at least 5%, e.g., from less than 85% to greater than 90%,during the adjustment phase.

It has now been discovered that benefits are derived by treating certainselectivated catalysts with a dealuminizing agent in accordance with thepresent disclosure. More particularly, when such dealuminizing agenttreated catalysts are used, the duration of the adjustment phase (i.e.,the time it takes to reach the target para-selectivity) is reduced.

The dealuminizing agents described herein include the various liquidsubstances known in the art for removing aluminum from aluminosilicatezeolites. Examples of dealuminizing agents include acids, chelatingagents and fluorosilicates. Acid dealuminizing agents include mineralacids, such as hydrochloric acid and nitric acid, and polyvalent acids,such as dicarboxylic acids. Acetic acid and formic acid are particularexamples of dealuminizing agents.

Suitable dicarboxylic acids include oxalic, malonic, succinic, glutaric,adipic, maleic, phthalic, isophthalic, terephthalic, fumaric, tartaricor mixtures thereof. Oxalic acid is preferred. The dicarboxylic acid maybe used in solution, such as an aqueous dicarboxylic acid solution.

Generally, the acid solution has a concentration in the range from about0.01 to about 4M. Preferably, the acid solution concentration is in therange from about 1 to about 3M.

The dicarboxylic acid may be in a volume solution to volume catalystratio of at least about 1:1, preferably at least about 4:1.

Treatment time with the dicarboxylic acid solution is as long asrequired to provide the desired dealumination. The treatment time may beat least about 10 minutes. Preferably, the treatment time is at leastabout 1 hour.

More than one dicarboxylic acid treatment step may be employed in theprocess of the present invention for enhanced dealumination.

The dicarboxylic acid treatment temperature may be in the range fromabout 32° F. to about reflux. For example, the treatment temperature maybe from about 60° F. to about 200° F., e.g., from about 120° F. to about180° F.

Examples of dealuminizing agents, which are chelating agents, includeurea, triethylenediamine and ethylenediaminetetraacetic acid.

Fluorosilicate dealuminizing agents are discussed in the Breck,deceased, et al., U.S. Pat. No. 4,503,023. A particular example of sucha fluorosilicate is (NH₄)SiF₆, especially in the form of an aqueoussolution thereof having a pH of from about 3 to about 7.

The parent zeolite, which is subjected to the selectivation treatmentdescribed herein, is preferably an intermediate pore size zeolite. Suchintermediate pore size zeolites may have a Constraint Index of betweenabout 1 and 12. A method for determining Constraint Index is describedin U.S. Pat. No. 4,016,218. Examples of zeolites which have a ConstraintIndex from about 1 to 12 include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23,ZSM-35, ZSM-48, ZSM-50 and ZSM-57. An especially preferred zeolite isZSM-5. Such zeolites are described, for example, in U.S. Pat. Nos.3,702,886; Re. 29,949; 3,709,979; 3,832,449; 4,046,859; 4,556,447;4,076,842; 4,016,245; 4,397,827; 4,640,849; 4,046,685; 4,175,114;4,199,556; 4,341,7448; 3,308,069; and Re. No. 28,341.

Zeolites, such as ZSM-5, may be selectivated with a siliceous materialby a vapor phase process or a liquid phase process. An example of aliquid phase selectivation process is described herein as apreselectivation or ex situ selectivation process. The preselectivationtreatment involves depositing siliceous material on the catalyst by thesteps of:

(a) combining a zeolite with an organosilicon compound; and

(b) calcining the organosilicon-containing material in anoxygen-containing atmosphere under conditions sufficient to removeorganic material therefrom and leave the siliceous material on thezeolite.

Examples of preselectivation techniques are provided in copending U.S.application Ser. Nos. 08/069,251, now U.S. Pat. No. 5,476,823;08/069,254; 08/069,255; and 08/069,259, each filed May 28, 1993. Ser.No. 08/069,254 is now U.S. Pat. No. 5,367,099; Ser. No. 08/069,255 isnow U.S. Pat. No. 5,404,800; and Ser. No. 08/069,259 is now U.S. Pat.No. 5,365,004.

The preselectivation treatment may result in the deposition of at least1 wt % of siliceous material on the catalyst.

A zeolite may be combined with a binder material for the zeolite. Thisbinder material is preferably an inert, non-alumina binder material,such as a silica binder. A zeolite may be subjected to one or moreselectivation treatments after the zeolite is combined with the bindermaterial. Optionally, however, the zeolite may be selectivated in theunbound state.

Procedures for preparing silica bound zeolites, such as ZSM-5, aredescribed in U.S. Pat. Nos. 4,582,815; 5,053,374; and 5,182,242. Aparticular procedure for binding ZSM-5 with a silica binder involves anextrusion process.

A particular process for preparing a silica-bound zeolite may comprisethe steps of:

(a) mulling and then extruding a mixture comprising water, zeolite,colloidal silica and sodium ions under conditions sufficient to form anextrudate having an intermediate green strength sufficient to resistattrition during ion exchange step (b) set forth hereinafter;

(b) contacting the uncalcined extrudate of step (a) with an aqueoussolution comprising ammonium cations under conditions sufficient toexchange cations in said zeolite with ammonium cations; and

(c) calcining the ammonium exchanged extrudate of step (b) underconditions sufficient to generate the hydrogen form of said zeolite andincrease the crush strength of said extrudate.

In accordance with examples of a preselectivation technique, thecatalyst may be preselectivated by single or multiple treatments with aliquid organosilicon compound in a liquid carrier, each treatment beingfollowed by calcination of the treated material in an oxygen-containingatmosphere, e.g., air.

In accordance with the multiple impregnation preselectivation method,the zeolite is treated at least twice, e.g., at least 3 times, e.g.,from 4 to 6 times, with a liquid medium comprising a liquid carrier andat least one liquid organosilicon compound. The organosilicon compoundmay be present in the form of a solute dissolved in the liquid carrieror in the form of emulsified droplets in the liquid carrier. For thepurposes of the present disclosure, it will be understood that anormally solid organosilicon compound will be considered to be a liquid(i.e., in the liquid state) when it is dissolved or emulsified in aliquid medium. The liquid carrier may be water, an organic liquid or acombination of water and an organic liquid. Particularly when the liquidmedium comprises an emulsion of the organosilicon compound in water, theliquid medium may also comprise an emulsifying agent, such as asurfactant.

Stable aqueous emulsions of organosilicon compounds (e.g., silicone oil)are described in copending U.S. application Ser. No. 08/141,758, filedOct. 27, 1993 now abandoned. These emulsions are generated by mixing theorganosilicon oil and an aqueous component in the presence of asurfactant or surfactant mixture. Useful surfactants include any of alarge variety of surfactants, including ionic and non-ionic surfactants.Preferred surfactants include non-nitrogenous non-ionic surfactants suchas alcohol, alkylphenol, and polyalkoxyalkanol derivatives, glycerolesters, polyoxyethylene esters, anhydrosorbitol esters, ethoxylatedanhydrosorbitol esters, natural fats, oils, waxes and ethoxylated estersthereof, glycol esters, polyalkylene oxide block co-polymer surfactants,poly(oxyethylene-co-oxypropylene) non-ionic surfactants, and mixturesthereof. More preferred surfactants include surfactants having theformulaα-[4-(1,1,3,3-tetramethylbutyl)phenyl]-ω-hydroxypoly(oxy-1,2-ethanediyl)(Octoxynols), most preferably octoxynol-9. Such preferred surfactantsinclude the TRITON® X series, such as TRITON® X-100 and TRITON® X-305,available from Rohm & Haas Co., Philadelphia, Pa., and the Igepal CAseries from GAF Corp., New York, N.Y. Emulsions formulated using suchsurfactants are effective for selectivating ZSM-5 to enhance shapeselectivity, and to passivate surface acidity detrimental to productselectivity in certain regioselective catalytic applications such as thedisproportionation of alkylbenzenes.

The organosilicon compound preselectivating agent may be, for example, asilicone, a siloxane, a silane or mixtures thereof. These organosiliconcompounds may have at least 2 silicon atoms per molecule. Theseorganosilicon compounds may be solids in pure form, provided that theyare soluble or otherwise convertible to the liquid form upon combinationwith the liquid carrier medium. The molecular weight of the silicone,siloxane or silane compound employed as a preselectivating agent may bebetween about 80 and about 20,000, and preferably within the approximaterange of 150 to 10,000. Representative preselectivation siliconecompounds include dimethyl silicone, diethyl silicone, phenylmethylsilicone, methylhydrogen silicone, ethylhydrogen silicone,phenylhydrogen silicone, methylethyl silicone, phenylethyl--silicone,diphenyl silicone, methyltrifluoropropyl silicone, ethyltrifluoropropylsilicone, polydimethyl silicone, tetrachlorophenylmethyl silicone,tetrachlorophenylethyl silicone, tetrachlorophenylhydrogen silicone,tetrachlorophenylphenyl silicone, methylvinyl silicone, and ethylvinylsilicone. The preselectivating silicone, siloxane or silane compoundneed not be linear, but may be cyclic, for example, hexamethylcyclotrisiloxane, octamethyl cyclotetrasiloxane, hexaphenylcyclotrisiloxane and octaphenyl cyclotetrasiloxane. Mixtures of thesecompounds may also be used as preselectivating agents, as may siliconeswith other functional groups.

Preferred organosilicon preselectivating agents, particularly when thepreselectivating agent is dissolved in an organic carrier or emulsifiedin an aqueous carrier, include dimethylphenylmethyl polysiloxane (e.g.,Dow-550) and phenylmethyl polysiloxane (e.g., Dow-710). Dow-550 andDow-710 are available from Dow Chemical Co., Midland, Mich.

When the organosilicon preselectivating agent is present in the form ofa water soluble compound in an aqueous solution, the organosilicon maybe substituted with one or more hydrophilic functional groups ormoieties, which serve to promote the overall water solubility of theorganosilicon compound. These hydrophilic functional groups may includeone or more organoamine groups, such as --N(CH₃)₃, --N(C₂ H₅)₃ and--N(C₃ H₇)₃. A preferred water soluble organosilicon preselectivatingagent is an n-propylamine silane, available as Hydrosil 2627 from HulsAmerica.

When the zeolite is preselectivated by a single or multiple impregnationtechnique, the zeolite is calcined after each impregnation to remove thecarrier and to convert the liquid organosilicon compound to a solidresidue material thereof. This solid residue material is referred toherein as a siliceous solid material, insofar as this material isbelieved to be a polymeric species having a high content of siliconatoms in the various structures thereof. However, this siliceous solidresidue material may also comprise carbon atoms in the structurethereof, resulting from the residue of the organo portion of theorganosilicon compound used to impregnate the catalyst.

Following each impregnation, the zeolite may be calcined at a rate offrom about 0.2° C./minute to about 5° C./minute to a temperature greaterthan 200° C., but below the temperature at which the crystallinity ofthe zeolite is adversely affected. This calcination temperature may bebelow 700° C., e.g., within the approximate range of 350° C. to 550° C.The duration of calcination at the calcination temperature may be from 1to 24 hours, e.g., from 2 to 6 hours.

The impregnated zeolite may be calcined in an inert or oxidizingatmosphere. An example of such an inert atmosphere is a nitrogen, i.e.,N₂, atmosphere. An example of an oxidizing atmosphere is anoxygen-containing atmosphere, such as air. Calcination may take placeinitially in an inert, e.g., N₂, atmosphere, followed by calcination inan oxygen-containing atmosphere, such as air or a mixture of air and N₂.Calcination should be performed in an atmosphere substantially free ofwater vapor to avoid undesirable uncontrolled steaming of the zeolite.The zeolite may be calcined once or more than once following eachimpregnation. The various calcinations following each impregnation neednot be identical, but may vary with respect to the temperature, the rateof temperature rise, the atmosphere and the duration of calcination.

The amount of siliceous residue material which is deposited on thezeolite or bound zeolite is dependent upon a number of factors includingthe temperatures of the impregnation and calcination steps, theconcentration of the organosilicon compound in the carrying medium, thedegree to which the catalyst has been dried prior to contact with theorganosilicon compound, the atmosphere used in the calcination andduration of the calcination.

Preferably, the kinetic diameter of both the organosilicon compound,which is used to preselectivate the zeolite, and the organosiliconcompound (e.g., silicone compound), which is used to functionalize thezeolite, is larger than the zeolite pore diameter, in order to avoidentry of the organosilicon compound into the zeolite pores and anyconcomitant reduction in the internal activity of the zeolite.

Vapor phase processes for selectivating zeolites are described incopending U.S. application Ser. Nos. 08/223,383, filed Feb. 25, 1993 nowabandoned; 08/233,542, filed May 5, 1994 now U.S. Pat. No. 5,475,179;08/306,567, filed Sep. 15, 1994; and 08/306,566, filed Sep. 15, 1994 nowU.S. Pat. No. 5,516,736.

The organosilicon compound which is used to vapor phase selectivate thezeolite may be a silicone or a silane. Silicones are defined herein asthose compounds wherein silicon atoms are bonded to one another viaoxygen atoms. Silanes are defined herein as those compounds whereinsilicon atoms are bonded directly to one another.

The silicone compound which may be used to selectivate the presentzeolite may be considered to be constructed of a siloxy backbonestructure capped with terminal groups. This siloxy backbone structuremay be a chain structure represented by the formula ##STR1## where p isfrom 1 to 9. This siloxy backbone structure may also be a cyclicstructure represented by the formula ##STR2## where q is from 2 to 10.Branched chain structures and composite chain/cyclic structures are alsopossible for the siloxy backbone of the silicone selectivating agent.

The hydrocarbyl groups which cap the available bonds of the siloxybackbone may have from 1 to 10 carbon atoms. Examples of suchhydrocarbyl groups are methyl and phenyl.

Examples of silicone compounds having a chain siloxy backbone structureinclude those of the formula ##STR3## where R₁ and R₆ are independentlyhydrogen, methyl, or phenyl; R₂, R₃, R₄, and R₅ are independently methylor phenyl; and m is from 1 to 10, e.g., from 1 to 4. Preferably, no morethan one phenyl group is bonded to each silicon atom. Particularexamples of such silicone compounds having a chain siloxy backbonestructure include hexamethyldisiloxane, decamethyltetrasiloxane anddiphenyltetramethyldisiloxane. Particular examples of silicone compoundshaving a cyclic siloxy backbone structure includeoctamethylcyclotetrasiloxane and decamethylcyclopentasiloxane.Particular examples of silicone compounds having a branched siloxybackbone structure are tris-(trimethylsiloxy)-phenylsilane andtris-(trimethylsiloxy)-silane.

The silane compounds, useful as vapor phase selectivating agentsaccording to the present method, may have structures corresponding tothe above-mentioned silicone compounds, wherein the silicon atoms arebonded directly to one another instead of via oxygen atoms. Examples ofsilanes having a chain backbone structure include those of the formula##STR4## where R₁ and R₆ are independently hydrogen, methyl, or phenyl;R₂, R₃, R₄, and R₅ are independently methyl or phenyl; and m is from 1to 10, e.g., from 1 to 4. An example of such a silane compound ishexamethyldisilane.

The vapor phase treatment is believed to result in the generation offunctionalized zeolites, thereby serving to selectivate the zeolite forcatalyzing certain reactions such as the disproportionation of toluene.Accordingly, the present vapor phase selectivation treatment is alsoreferred to herein as a functionalization treatment.

The present zeolite may be selectivated by more than one selectivationmethod, including those which are distinguished from the presentselectivation method. In particular, prior to contact with the presentvapor phase silicone functionalizing agent, the zeolite may be contactedwith a liquid phase including an organosilicon compound, followed bycalcination in an oxygen-containing atmosphere. Such a pretreatment ofthe zeolite is referred to herein as a preselectivation treatment.

In accordance with the present vapor phase selectivation orfunctionalization method described herein, the zeolite is contacted witha feed stream comprising a silicone or silane compound under vapor phaseconditions. The silicone or silane compound may be applied to thezeolite neat (i.e., in the absence of a carrier or other cofeed) by achemical vapor deposition technique. This feed stream may also comprisehydrogen and/or an organic carrier. Vapor phase conditions may include atemperature ranging from about 100° C. to about 600° C., e.g., fromabout 300° C. to about 500° C. When the silicone or silane compound isapplied neat, reduced pressures, e.g., from about 0.5 Torr to less thanatmospheric, may be used. Preferably, however, the silicone or silanecompound is applied along with cofed hydrogen (i.e., H₂) and an organiccarrier. In general, vapor phase conditions may include a pressureranging from about 0 to about 2000 psig, e.g., from about 15 to about800 psig, a mole ratio of hydrogen to hydrocarbons (e.g., toluene) fromabout 0.1 to 20, e.g., from about 0.1 to 10, e.g., from about 1 to about4, and a weight hourly space velocity (WHSV) from about 0.1 to about 100hr⁻¹, e.g., from about 0.1 to about 10 hr⁻¹. The organic carrier may bea hydrocarbon, especially an aromatic hydrocarbon such as toluene,benzene, xylenes and trimethylbenzenes. Toluene may comprise about 50 wt% to 100 wt %, e.g., at least 80 wt %, of the hydrocarbons in thefeedstock.

When a reactive hydrocarbon, such as toluene is included in thefeedstock, the presence of a sufficient amount of hydrogen in theselectivation feedstock is necessary to prevent rapid aging of thecatalyst during the selectivation process resulting in an excessivereduction in the zeolite activity, possibly accompanied by a reductionin toluene disproportionation selectivity to para-xylene. This rapidaging is believed to result from a rapid build-up of excessive amountsof carbonaceous deposits (i.e., coke), which may even extend into thepore system of the zeolite in the catalyst. However, even when hydrogenis used in optimal fashion to prevent aging during the selectivationprocess, a small amount of carbonaceous deposit may form on thecatalyst. The presence of hydrogen may also serve to induce or enhancethe chemical reaction between the zeolite and the selectivating agent,which results in the functionalization of the zeolite. This chemicalreaction is also believed to be induced or enhanced by elevated contacttemperatures, which may be needed to maintain the silicone or silanefunctionalizing agent in the vapor phase.

Confirmation of the reaction between the zeolite and the silicone orsilane compound may be made by an appropriate analysis of the zeoliteafter the reaction, as well as by monitoring and analyzing the off-gasesproduced by the reaction. Analysis of the zeolite will indicate thepresence of hydrocarbyl groups incorporated onto the zeolite from theorganosilicon selectivating agent. When the functionalized zeolite isused as a catalyst in an organic conversion process, these hydrocarbylgroups may remain intact on the zeolite. More particularly, one mayintentionally avoid the customary practice of precalcining the zeolite,prior to the organic conversion process, under conditions sufficient todecompose and/or burn off organic residue on the catalyst. Suchprecalcination conditions to be avoided may include contact of thezeolite at temperatures greater than 300° C. in an oxygen-containingatmosphere, e.g., air.

Selectivation of the zeolite may occur, in-situ, during the course of anorganic conversion reaction catalyzed by the zeolite, by including anorganosilicon selectivating agent, optionally along with H₂, in the feedto the organic conversion reaction. This type of in-situ selectivationis also referred to herein as trim selectivation.

The selectivated zeolite is a catalyst. This catalyst may be used aloneor in combination with other catalyst components included in catalystsof this type. Such other components include binders andhydrogenation/dehydrogenation components. Accordingly, it will beunderstood that the term, present catalyst, as used herein is intendedto connote the presently selectivated zeolite in combination with othercatalyst components, if any.

While not wishing to be bound by any theory, it is theorized that theextreme selectivity of the present catalyst is obtained by renderingacid sites on the external surfaces of the zeolite substantiallyinaccessible to reactants, while possibly increasing the tortuosity ofthe catalyst pore system. In a toluene disproportionation process usinga non-selectivated catalyst, acid sites existing on the externalsurfaces of the zeolite are believed to isomerize the productpara-xylene back to an equilibrium level with the other two xyleneisomers, thereby reducing the amount of para-xylene in the xylenes toonly about 24%. By reducing the availability of these external acidsites to the product para-xylene, it is theorized that a relatively highproportion of the para isomer can be retained. It is theorized thatexternal zeolite acid sites are blocked or otherwise unavailable topara-xylene in the present catalyst. The extreme para-selectivity of thepresent catalyst is especially surprising in the highly active forms ofthe catalyst.

The "alpha value" of a catalyst is an approximate indication of itscatalytic cracking activity. The alpha test is described in U.S. Pat.No. 3,354,078 and in the Journal of Catalysis, vol. 4, 522-529 (1965);vol. 6, 278 (1966); and Vol. 61, 395 (1980), each incorporated herein byreference to that description. It is noted that intrinsic rate constantsfor many acid-catalyzed reactions are proportional to the alpha valuefor a particular crystalline silicate catalyst (see "The Active Site ofAcidic Aluminosilicate Catalysts," Nature Vol 309, No 5959, 589-591,(1984)). The experimental conditions of the alpha test preferablyinclude a constant temperature of 538° C. and a variable flow rate asdescribed in detail in the Journal of Catalysis, vol. 61, 395 (1980).The present catalysts may have an alpha value greater than 50, e.g.,greater than 200, e.g., from about 200 to about 1500. The alpha value ofthe catalyst may be increased by mild steaming before selectivation.This type of steaming is discussed in U.S. Pat. No. 4,326,994.

The silica to alumina ratio of zeolites may be determined byconventional analysis. This ratio is meant to represent, as closely aspossible, the ratio in the rigid atomic framework of the zeolite crystaland to exclude silicon or aluminum in the binder or in cationic or otherform within the channels. The silica to alumina molar ratio of the ZSM-5used to prepare the present catalysts may be less than 100, e.g., lessthan 60, e.g., less than 40, e.g., from about 20 to about 40. It will beappreciated that it may be extremely difficult to directly measure thesilica to alumina ratio of zeolite after it has been combined with abinder material and selectivated by methods described hereinabove.Accordingly, the silica to alumina ratio has been expressed hereinabovein term of the silica to alumina ratio of the parent zeolite, i.e., thezeolite used to prepare the catalyst, as measured prior to theselectivation of the zeolite and prior to the combination of thiszeolite with the other catalyst components.

The crystal size of the parent zeolites of the present catalysts ispreferably greater than 0.1 microns, as calculated by methods describedhereinbelow. The accurate direct measurement of the crystal size ofzeolite materials is frequently very difficult. Microscopy methods, suchas SEM and TEM, may be used, but these methods require measurements of alarge number of crystals and, for each crystal measured, values may beevaluated in up to three dimensions. Furthermore, in order to morecompletely characterize the crystal size of a batch of crystals, oneshould calculate the average crystal size, as well as the degree ofvariance from this average in terms of a crystal size distribution.Rather than relying upon such complex evaluations, crystal size isexpressed herein in terms of a calculated value of average crystal sizeobtained by measuring the rate of sorption of 2,2-dimethylbutane at 90°C. and 60 torr hydrocarbon pressure. The crystal size is computed byapplying the diffusion equation given by J. Crank, The Mathematics ofDiffusion, Clarendon Press, 52-56 (1957), for the rate of sorbate uptakeby a solid whose diffusion properties can be approximated by a planesheet model. In addition, the diffusion constant of 2,2-dimethylbutane,D, under these conditions, is taken to be 1.5×10⁻¹⁴ cm² /sec. Therelation between crystal size measured in microns, d, and diffusion timemeasured in minutes, t₀.3, the time required for the uptake of 30%capacity of hydrocarbon, is:

    d=0.0704×t.sub.0.3.sup.1/2.

Particular measurements expressed herein were made on a computercontrolled, thermogravimetric electrobalance, but there are numerousways one skilled in the art could obtain the data. Examples of largercrystal material have a sorption time, t₀.3, of 497 minutes, which givesa calculated crystal size of 1.6 microns. Examples of smaller crystalmaterial have a sorption time of 7.8 minutes, and a calculated size of0.20 microns.

As pointed out in the aforementioned U.S. Pat. No. 4,117,026, largercrystal size zeolites tend to have a greater intrinsic para-selectivitythan smaller crystal size zeolites. It is theorized that this differenceis attributable to the smaller ratio of external surface area toavailable internal surface area for larger zeolites as compared tosmaller crystal size zeolites. Since it would theoretically require lessselectivation to neutralize the external surface area of the moreintrinsically para-selective larger crystal size zeolites, largercrystal size zeolites would be preferred to smaller crystal sizezeolites, provided that all other factors were equal. However, there areother factors which tend to mitigate against a preference for largercrystal size zeolites, particularly ZSM-5. More particularly, largercrystal size ZSM-5 having a high activity and corresponding low silicato alumina molar ratio, e.g., from about 20 to about 40, is considerablymore difficult to prepare than smaller crystal size ZSM-5, especially ona commercial scale. A particularly surprising aspect of the presentsiliceous material selectivated catalysts is that the zeolites thereofmay comprise relatively small crystal size ZSM-5, e.g., having a crystalsize of from about 0.1 to about 0.5 microns and a silica to aluminamolar ratio of from about 20 to 40, and still have an extremely highdegree of para-selectivity. When larger crystal size ZSM-5 is chosen forthe present catalyst, this the crystal size of this ZSM-5 may be, forexample, from about 1 to 2 microns.

The present catalyst is particularly adapted for the production ofpara-xylene via the catalytic disproportionation of toluene. Moreparticularly, this catalyst, under disproportionation conditions, iscapable of high conversions of toluene, while at the same time producinga very high proportion of para-xylene among the total of the xyleneisomers. However, it will be understood that this catalyst may also beused to catalyze other organic, especially hydrocarbon, conversionreactions.

When the present catalyst is used in a toluene disproportionationreaction, the reaction conditions may include a temperature of about350° C.-540° C., a pressure of about atmospheric -5000 psig, a toluenefeed rate of about 0.1-20 WHSV, and a hydrogen to toluene mole ratio ofabout 0.1-20. The hydrogen cofeed serves to suppress catalyst aging,thereby dramatically increasing the cycle length. The liquid feedstockfor the present toluene disproportionation reaction may, optionally,include hydrocarbons other than toluene. Such hydrocarbons includenon-aromatic hydrocarbons, such as paraffins and/or cycloparaffins.These non-aromatics may have boiling points close to the boiling pointof toluene, which is about 111° C. These non-aromatics are, therefore,difficult to remove from toluene by distillation, and extractiontechniques may be needed to separate these toluene coboilers fromtoluene. The amount of non-aromatics in the fresh feed may be from 0 wt.% to about 3 wt. %, e.g., from about 0.2 wt. % to about 1.5 wt. %. Itwill also be understood that commercial toluene disproportionationreactions are run by recycling unconverted toluene. The amount ofrecycled toluene in the feed to the reactor will vary on the amount oftoluene conversion per pass. For example, this feed may comprise fromabout 50 wt. % to about 85 wt. % of recycled toluene. As a result,difficult to remove non-aromatic constituents (e.g., toluene coboilers)may build up in the recycle stream. These toluene coboilers mayeventually comprise from about 2 wt. % to about 15 wt. % of the toluenerecycle stream. Thus, the total liquid feed to the presentdisproportionation reactor may comprise both fresh (i.e., make-up)toluene and recycled toluene, and this liquid feed may comprise from 0wt. % to about 15 wt. % of non-aromatics.

When the present catalyst is used in an ethylbenzene disproportionationreaction, the reaction conditions may include a temperature of about200° C. to about 600° C., e.g., from about 350° C. to about 540° C.; apressure of from about atmospheric to about 5000 psig, e.g., from about100 to about 1000 psig; an ethylbenzene feed rate of from about 0.1 WHSVto about 20 WHSV, e.g., from about 2 WHSV to about 10 WHSV; and ahydrogen to ethylbenzene mole ratio of from about 0.1 to about 20, e.g.,from about 2 to about 6.

The present catalysts may be used to convert paraffins from high to lowmolecular weight hydrocarbons in a dewaxing process. Examples of suchdewaxing processes are disclosed in U.S. Pat. Nos. 3,700,585; Re.28,398; 3,968,024; and 4,181,598, the entire disclosures of which areincorporated herein by reference. Hydrocarbon feeds for dewaxingprocesses include petroleum stocks which have a freeze point or pourpoint problem, e.g., petroleum stocks boiling above 350° F. Lubricatingoil stocks may be feedstocks to a dewaxing process. The dewaxing may becarried out under either cracking or hydrocracking conditions. Crackingconditions for dewaxing may include a liquid hourly space velocity(LHSV) between about 0.5 and 200, a temperature between about 288° C.(550° F.) and 590° C. (1100° F.), a pressure between aboutsubatmospheric and several hundred atmospheres. Hydrocracking conditionsfor dewaxing may include a liquid hourly space velocity (LHSV) betweenabout 0.1 and 10, a temperature between about 340° C. (650° F.) and 538°C. (1000° F.), a pressure between about 100 and 3000 psig, and ahydrogen to hydrocarbon mole ratio between about one and 20.

The present catalysts may be used to catalyze a variety of alkylaromaticconversion reactions, including isomerization reactions. Suchconversions include those described, for example, in U.S. Pat. Nos.3,856,872; 3,856,873; Re. 30,157; 4,101,595; 4,101,597; 4,312,790; Re.31,919; and 4,224,141, the entire disclosures of which are incorporatedby reference.

As per process conditions described in U.S. Pat. No. 3,856,872 toMorrison, the present catalyst may be used for catalyzing the conversionof C₈ aromatics, i.e., xylene and/or ethylbenzene, to para-xylene(octafining) at a temperature of 550° F. (288° C.) to 900° F. (482° C.),a pressure of 150 to 300 psig, and a liquid hourly space velocity (LHSV)of 1 to 200. When used in this reaction, the catalyst may comprise ahydrogenation metal, such as platinum or nickel, and the feed to thereaction may include hydrogen.

As per process conditions described in U.S. Pat. No. 3,856,873 toBurress, the present catalyst may be used for catalyzing the conversionof mixtures of C₈ aromatic hydrocarbons to para-xylene in a vapor phasereaction at a temperature of 500° F. (260° C.) to 1000° F. (538° C.), apressure of 0 (atmospheric) to 1000 psig, and a weight hourly spacevelocity (WHSV) of 0.5 to 250 with no added hydrogen.

As per process conditions described in U.S. Pat. No. 4,476,330 to Kerret al., the present catalyst may be used for catalyzing the conversionof aliphatic oxygenates to a higher molecular weight compound at atemperature of 70° F. (21° C.) to 1400° F. (760° C.). The feeds includelower aliphatic organic oxygenates having up to 6 carbon atoms. Theoxygenates may be selected from the group consisting of acetals, ketals,acid halides, alcohols, carboxylic acids, aldehydes, acid anhydrides,epoxides, ethers, hemiacetals, gem diols, hydroxy acids, ketones,ketenes, lactones, peracids, peroxides and sugars, especially alcohols,ethers and esters.

The present catalysts may be used as catalysts in the oligomerization ofolefins to form gasoline, distillate, lube oils and/or chemicals.Examples of such oligomerization processes are disclosed in U.S. Pat.Nos. 4,517,399; 4,520,221; 4,547,609; and 4,547,613, the entiredisclosures of which are incorporated herein by reference.

As per process conditions described in U.S. Pat. No. 4,517,399 toChester et al., the present catalyst may be used for catalyzing theconversion of olefins having from 3 to 18 carbon atoms, e.g., propylene,to high viscosity, low pour point lubricating oils. Conversionconditions may include a temperature of 350° F. (177° C.) to 650° F.(343° C.), a pressure of 100 to 5000 psig, and a weight hourly spacevelocity (WHSV) of 0.1 to 10.

The present catalysts may be used as catalysts in the conversion of avariety of aromatic compounds to provide dialkyl-benzene products whichare highly enriched in the para-dialkyl substituted benzene isomer.Conversion reactions of this type include aromatics alkylation,transalkylation and disproportionation. Examples of such aromaticalkylation processes are disclosed in U.S. Pat. Nos. 3,755,483;4,086,287; 4,117,024; and 4,117,026, the entire disclosures of which areincorporated herein by reference.

As per process conditions described in U.S. Pat. No. 3,755,483 toBurress, the present catalyst may be used for catalyzing the alkylationof aromatic hydrocarbons, such as benzene, naphthalene, anthracene andsubstituted derivatives thereof, e.g., toluene and xylene, withalkylating agents having 1 to 24 carbon atoms under vapor phaseconditions. The alkylating agents may be selected from the groupconsisting of olefins, such as ethylene, propylene and dodecene,aldehydes, such as formaldehyde, alkyl halides and alcohols. Conversionconditions may include an inlet temperature of up to about 900° F. (428°C.), with a reactor bed temperature of up to about 1050° F. (566° C.), apressure of about atmospheric to about 3000 psig, a ratio ofaromatic/alkylating agent of about 1:1 to about 20:1 and a weight hourlyspace velocity (WHSV) of 20 to 3000.

As per process conditions described in U.S. Pat. No. 4,086,287 toKaeding et al., the present catalyst may be used for catalyzing theethylation of toluene or ethylbenzene to produce a para-ethylderivative, e.g., para-ethyltoluene. Conversion conditions may include atemperature of from about 250° C. to about 600° C., a pressure of 0.1atmospheres to about 100 atmospheres, a ratio of aromatic/ethylatingagent of about 1:1 to about 10:1 and a weight hourly space velocity(WHSV) of 0.1 to 100.

The present catalysts may be used as catalysts in the conversion oflight paraffins and olefins to aromatic compounds. Examples of suchconversions are disclosed in U.S. Pat. Nos. 3,760,024 and 3,756,942, theentire disclosures of which are incorporated herein by reference.

As per process conditions described in U.S. Pat. No. 3,760,024 toCattanach, the present catalyst may be used for catalyzing theconversion of paraffins having 2 to 4 carbon atoms and/or olefins toaromatics having from 6 to 10 carbon atoms. The catalyst may,optionally, include a hydrogenation/dehydrogenation component.Conversion conditions may include a temperature of from about 100° C. toabout 650° C., a pressure of 0 to about 1000 psig, a ratio ofhydrogen/hydrocarbon of about 0 to about 20 and a weight hourly spacevelocity (WHSV) of 0.1 to 500.

The present catalysts may be used as catalysts in the synthesis ofpyridine and substituted pyridines. Process conditions may be selectedfrom those disclosed in U.S. Pat. Nos. 4,675,410 and 4,220,783, theentire disclosures of which are incorporated herein by reference.

The present catalysts may be used as catalysts in the synthesis ofcaprolactam by the Beckmann rearrangement of cyclohexane oxime. Processconditions may be selected from those disclosed in U.S. Pat. No.4,359,421, the entire disclosures of which are incorporated herein byreference.

Accordingly, it will be understood that the present catalysts may beused to catalyze a variety of organic, e.g., hydrocarbon, conversionprocesses. Examples of such processes include cracking hydrocarbons withreaction conditions including a temperature of from about 300° C. toabout 700° C., a pressure of from about 0.1 atmosphere (bar) to about 30atmospheres and a weight hourly space velocity of from about 0.1 hr⁻¹ toabout 20 hr⁻¹ ; dehydrogenating hydrocarbon compounds with reactionconditions including a temperature of from about 300° C. to about 700°C., a pressure of from about 0.1 atmosphere to about 10 atmospheres andweight hourly space velocity of from about 0.1 to about 20; convertingparaffins to aromatics with reaction conditions including a temperatureof from about 300° C. to about 700° C., a pressure of from about 0.1atmosphere to about 60 atmospheres, a weight hourly space velocity offrom about 0.5 to about 400 and a hydrogen/hydrocarbon mole ratio offrom about 0 to about 20; converting olefins to aromatics, e.g.,benzene, toluene and xylene, with reaction conditions including atemperature of from about 100° C. to about 700° C., a pressure of fromabout 0.1 atmosphere to about 60 atmospheres, a weight hourly spacevelocity of from about 0.5 to about 400 and a hydrogen/hydrocarbon moleratio of from about 0 to about 20; converting alcohols, e.g., methanol,or ethers, e.g., dimethylether, or mixtures thereof to hydrocarbonsincluding olefins and/or aromatics with reaction conditions including atemperature of from about 275° C. to about 600° C., a pressure of fromabout 0.5 atmosphere to about 50 atmospheres and a liquid hourly spacevelocity of from about 0.5 to about 100; isomerizing xylene feedstockcomponents with reaction conditions including a temperature of fromabout 230° C. to about 510° C., a pressure of from about 3 atmospheresto about 35 atmospheres, a weight hourly space velocity of from about0.1 to about 200 and a hydrogen/hydrocarbon mole ratio of from about 0to about 100; disproportionating toluene with reaction conditionsincluding a temperature of from about 200° C. to about 760° C., apressure from about atmospheric to about 60 atmospheres and a weighthourly space velocity of from about 0.08 to about 20; alkylatingaromatic hydrocarbons, e.g., benzene and alkylbenzenes in the presenceof an alkylating agent, e.g., olefins, formaldehyde, alkyl halides andalcohols, with reaction conditions including a temperature of from about250° C. to about 500° C., a pressure of from about atmospheric to about200 atmospheres, a weight hourly space velocity of from about 2 to about2000 and an aromatic hydrocarbon/alkylating agent mole ratio of fromabout 1/1 to about 20/1; and transalkylkating aromatic hydrocarbons inthe presence of polyalkylaromatic hydrocarbons with reaction conditionsincluding a temperature of from about 340° C. to about 500° C., apressure of from about atmospheric to about 200 atmospheres, a weighthourly space velocity of from about 10 to about 1000 and an aromatichydrocarbon/polyalkylaromatic hydrocarbon mole ratio of from about 1/1to about 16/1.

In general, therefore, catalytic conversion conditions over the presentcatalyst may include a temperature of from about 100° C. to about 760°C., a pressure of from about 0.1 atmosphere (bar) to about 200atmospheres (bar), a weight hourly space velocity of from about 0.08hr⁻¹ to about 2000 hr⁻¹ and a hydrogen/organic, e.g., hydrocarboncompound, of from 0 to about 100.

The present catalyst may, optionally, include a binder material, Theoptional binder for the present catalyst is preferably an inert,non-alumina-containing material, such as silica. However, the binder mayalso be selected from other materials which may be used exclusively orin combination with one another or with silica. Examples of such bindermaterials include alumina, zirconia, magnesia, titania, thoria andboria. These materials may be used in the form of dried inorganic oxidegels of gelatinous precipitates. Examples of clay binder materialsinclude bentonite and kieselguhr. The relative proportion of zeolite tothe binder material may be about 30 to about 90 percent by weight. Thebound catalyst may be in the form of an extrudate, beads or fluidizablemicrospheres.

Optionally, the present catalyst may contain ahydrogenation/dehydrogenation component. Examples of such optionalcomponents include the oxide, hydroxide or free metal (i.e., zerovalent) forms of Group VIII metals (i.e., Pt, Pd, Ir, Rh, Os, Ru, Ni, Coand Fe), Group IVA metals (i.e., Sn and Pb), Group VB metals (i.e., Sband Bi), and Group VIIB metals (i.e., Mn, Tc and Re). Noble metals(i.e., Pt, Pd, Ir, Rh, Os and Ru) are particular optionalhydrogenation/dehydrogenation components. Combinations of catalyticforms of such noble or non-noble metal, such as combinations of Pt withSn, may be used. The valence state of the metal is preferably in areduced valence state, e.g., when this component is in the form of anoxide or hydroxide. The reduced valence state of this metal may beattained, in situ, during the course of a reaction, when a reducingagent, such as hydrogen, is included in the feed to the reaction.Preferably, the present catalyst is free of noble metal.

The optional hydrogenation/dehydrogenation component may be incorporatedinto the catalyst by methods known in the art, such as ion exchange,impregnation or physical admixture. For example, solutions ofappropriate metal salts may be contacted with the remaining catalystcomponents, either before or after selectivation of the catalyst, underconditions sufficient to combine the respective components. Themetal-containing salt is preferably water soluble. Examples of suchsalts include chloroplatinic acid, tetrammineplatinum complexes,platinum chloride, tin sulfate and tin chloride.

The amount of optional hydrogenation/dehydrogenation component may bethat amount which imparts or increases the catalytic ability of theoverall catalyst to catalytically hydrogenate or dehydrogenate anorganic compound under sufficient hydrogenation or dehydrogenationconditions. This amount is referred to herein as a catalytic amount.Quantitatively speaking, when the present catalyst comprises a noblemetal, it may comprise, for example, from about 0.001 to about 5 wt %,e.g., from about 0.1 to about 2 wt %, of the noble metal.

EXAMPLE 1

The catalysts used in this study, ACC3 and ACC4, were prepared bymultiply treating (via ex situ silica selectivation) ZSM-5 catalysts.ACC3 was prepared by treating a base 65% ZSM-5/35% silica binder withthree successive treatments with 7.8 wt. % Dow-550 in decane, while ACC4was prepared via four successive treatments. In brief, the procedureused for catalyst preparation was a multiple silcone coating procedure(pore filling technique) with Dow-550/decane. After impregnation, thesolvent was stripped, and the catalyst was calcined in N₂ and then inair to 538° C.

EXAMPLE 2

The toluene disproportionation evaluations were conducted in automatedunits with on-line G.C. sampling. Two grams of catalyst was loaded intoa 0.25 inch diameter, stainless steel tube reactor (with sand as aninert packing material). The sample was heated in N₂ to reactiontemperature, and the toluene/hydrogen feed was then introduced. Furtherdetails on the conditions of reaction are as summarized below. The feedwas alumina-percolated toluene.

EXAMPLE 3

A 1 molar solution of oxalic acid was prepared by dissolving 126.07grams of oxalic acid dihydrate (1 mole) in 1 liter of distilled water.This solution was stirred and gently heated until the oxalic acidcompletely dissolved. Twenty grams of ACC4 was loaded into a 1 literround bottom flask. Eight hundred mL of 1 molar oxalic acid solution wasthen added to this flask. The flask was then heated to 45° C. and heldat this temperature for 1 hour, then cooled to room temperature. Theextrudates were then recovered by filtration, washed with 500 mL ofdistilled water, air dried, and transferred to a crucible. The cruciblewas then placed in an oven where the extrudates were dried for 6 hoursat 120° C. and calcined at 450° C. for 2 hours.

EXAMPLE 4

A 1 molar solution of oxalic acid was prepared by dissolving 126.07grams of oxalic acid dihydrate (1 mole) in 1 liter of distilled water.This solution was stirred and gently heated until the oxalic acidcompletely dissolved. Three hundred mL of 1M oxalic acid was added toapproximately 10 grams of ACC3 in a 500 cc Erlenmeyer flask. Afterapproximately 20 minutes, the solution was filtered, and the extrudateswere washed with approximately 200 mL of distilled water. The extrudateswere then placed in a 500 cc Erlenmeyer flask, and 200 mL of 1M oxalicacid was then added to this flask. The solution was again allowed to sitfor 20 minutes. The extrudates were collected by filtration, washed with500 mL distilled water, and air dried. These extrudates were then placedin a quartz crucible, dried at 120° C. for 4 hours and calcined at 538°C. for 3 hours.

EXAMPLE 5

A fresh sample of ACC3 and a sample of the treated ACC3 (as prepared inExample 4) were then evaluated using the conditions of 3 WHSV, 1 H₂ /HC,300 psig. The results are summarized below in Table 1.

                  TABLE 1                                                         ______________________________________                                                           Oxalic Acid-                                                            ACC3  Treated ACC3                                               ______________________________________                                        Time (hrs.)    17      23                                                     Temp (°F.)                                                                            734     741                                                    Pressure (psig)                                                                              277     270                                                    WHSV (1/hr)    3       3                                                      H.sub.2 /HC    1       1                                                      Yields (wt. %)                                                                C.sub.5 -      0.7     0.7                                                    Benzene        13.1    13.3                                                   Ethylbenzene   0.3     0.3                                                    Pars-xylene    7.3     8.8                                                    Meta-xylene    5.9     5.2                                                    Ortho-xylene   1.1     1.0                                                    Toluene        29.3    30.2                                                   Conc. (%)                                                                     Para-selec-    50.6    58.6                                                   tivity (%)                                                                    ______________________________________                                    

The results clearly show that the para-selectivity and para-xylene yieldare improved by treating ACC3 with oxalic acid.

EXAMPLE 6

A fresh sample of ACC4 and a sample of the treated ACC4 (as prepared inExample 3) were then evaluated using the conditions of 3 WHSV, 1 H₂ /HC,and 300 psig. The results are summarized below in Table 2.

                  TABLE 2                                                         ______________________________________                                                           Oxalic Acid-                                                            ACC4  Treated ACC4                                               ______________________________________                                        Time (hrs.)    15      13                                                     Temp (°F.)                                                                            753     734                                                    Pressure (psig)                                                                              281     277                                                    WHSV (1/hr)    3       3                                                      H.sub.2 /HC    1       1                                                      Yields (wt. %)                                                                C.sub.5 -      0.9     1.0                                                    Benzene        13.1    12.9                                                   Ethylbenzene   0.4     0.4                                                    Pars-xylene    12.3    13.0                                                   Meta-xylene    2.2     1.4                                                    Ortho-xylene   0.3     0.2                                                    Toluene        30.0    29.7                                                   Conc. (%)                                                                     Para-selec-    82.9    88.7                                                   tivity (%)                                                                    ______________________________________                                    

The results clearly show that the para-selectivity and para-xylene yieldare improved by treating ACC4 with oxalic acid.

EXAMPLE 7

A fresh sample of ACC4 and a sample of the treated ACC4 (as prepared inExample 3) were then evaluated using the conditions of 3 WHSV, 1 H₂ /HC,and 270 psig. The results are summarized below in Table 3.

                  TABLE 3                                                         ______________________________________                                                      ACC4              ACC4                                          Treatment     None              Oxalic Acid                                   ______________________________________                                        Temp (°F.)                                                                           741    751        739  749                                      TOS (hrs.)    28     125        25   133                                      Tol. conv. (%)                                                                              29.5   28.5       29.6 29.6                                     Para-selec-   84.0   90.3       88.6 90.0                                     tivity (%)                                                                    Yields (wt. %)                                                                C.sub.5 -     0.9    1.0        0.8  1.1                                      Xylenes       14.4   13.5       14.6 13.9                                     Pars-xylene   12.1   12.2       13.0 12.5                                     ______________________________________                                    

EXAMPLE 8

Severe catalyst handling, as simulated by the ASTM certified methodD-4098 (rolling baffle can test), either at standard evaluation times(1/2 hour) or at 48 times this normal severity (24 hours), has beenobserved to result in a decrease in para-selectivity of 6% and 12%,respectively, for ACC4.

This Example demonstrates that the present oxalic acid treatmentimproves the performance of attrited catalysts, decreasing theselectivity loss associated with catalyst attrition.

Attrited ACC4 was treated with oxalic acid in the manner of Example 3.Toluene disproportionation performance was measured as a function ofattrition severity and as a function of post-attrition oxalic acidtreatment. Conditions included 3 WHSV, 1 H₂ /HC, and 270 psig. Theresults are summarized below in Table 4.

                  TABLE 4                                                         ______________________________________                                                ACC4             ACC4                                                 ______________________________________                                        Attrition Time                                                                          None     1/2      24     1/2   24                                   (hrs)                                                                         Treatment None     None     None   Oxalic                                                                              Acid                                 Temp (°F.)                                                                       741      743      754    741   741                                  TOS (hrs.)                                                                              28       26       28     29    28                                   Tol. conv. (%)                                                                          29.5     29.3     30.0   29.4  29.2                                 Para-selec-                                                                             84.0     79.1     72.0   89.9  85.9                                 tivity (%)                                                                    Yields (wt. %)                                                                C.sub.5 - 0.9      0.9      0.9    0.9   0.8                                  Xylenes   14.4     14.1     14.5   14.4  14.7                                 Pars-xylene                                                                             12.1     11.2     10.4   12.9  12.6                                 ______________________________________                                    

What is claimed is:
 1. A method for preparing a selectivated catalystcomposition, said method comprising the steps of:(a) combining anintermediate pore size zeolite with an organosilicon compound; (b)calcining the organosilicon-containing material in an oxygen-containingatmosphere under conditions effective to remove organic materialtherefrom and deposit a siliceous material on said catalyst; (c)contacting the catalyst comprising a zeolite and siliceous material fromstep (b) with an aqueous solution comprising at least one dealuminizingagent; (d) washing the aqueous solution treated catalyst from step (c)with water; (e) drying the washed catalyst from step (d); and (e)calcining the dried catalyst from step (d) under conditions sufficientto remove any residue of said dealuminizing agent; the catalyst fromstep (e) having a greater para selectivity and para xylene yield that acatalyst not treated with said dealuminizing agent.
 2. A methodaccording to claim 1, wherein said dealuminizing agent is a monovalentacid.
 3. A method according to claim 2 wherein said monovalent acid isselected from the group consisting of acetic acid, formic acid, nitricacid and hydrochloric acid.
 4. A method according to claim 1, whereinsaid dealuminizing agent is a chelating agent.
 5. A method according toclaim 4, wherein said chelating agent is selected from the groupconsisting of triethylenediamine, urea and ethylenediaminetetraaceticacid.
 6. A method according to claim 1, wherein said dealuminizing agentis a polyvalent acid.
 7. A method according to claim 6, wherein saidchelating agent is oxalic acid.
 8. A method according to claim 1,wherein said dealuminizing agent is a fluorosilicate.
 9. A methodaccording to claim 8, wherein said fluorosilicate is (NH₄)₂ SiF₆.
 10. Amethod according to claim 1, wherein said zeolite is ZSM-5.
 11. A methodaccording to claim 10, wherein said ZSM-5 has a silica to alumina molarratio of 60 or less.
 12. A method according to claim 1, wherein saidcatalyst comprises at least 1 wt % of said siliceous materialselectivating agent.
 13. A method according to claim 1, wherein saidcatalyst further comprises a binder material.
 14. A method according toclaim 13, wherein said binder material is silica.
 15. A method accordingto claim 1, wherein step (a) is repeated at least once.
 16. A methodaccording to claim 1, wherein steps (b) and (c) are repeated at leastonce prior to conducting step (d).